Thermocouple vacuum gauges serve several functions in a cryopump controller, none of which requires a high degree of accuracy in the pressure measurement. The thermocouple is used during the evacuation of the cryopump from atmospheric pressure, either during an initial evacuation or when a cryopump is regenerated, to determine when to start the cryopump.
During evacuation the air is quickly removed from the cryopump but an extended period of time is typically required to remove water vapor that has been adsorbed on the cryopump walls and in the charcoal adsorbent that is typically used in the second stage cryopanel. In many systems, the cryopump is turned on when the thermocouple or other vacuum gauge reads a pressure of between about 0.1 and 0.2 torr. It is common practice to take readings of the rate of pressure rise before the pump is turned on to determine if the cryopump is “clean” or if there is a leak.
Once a cryopump is cold the pressure will be much lower than 0.001 torr and the thermocouple gauge gives a minimum reading. After a power interruption the pressure may rise above 0.001 torr. This eventuality may be included in the logic to decide if the cryopump should be restarted or put into a regeneration mode.
Thermocouples are commonly used temperature sensors as they are rugged, inexpensive and operate over a wide temperature range. Thermocouples are based on the Seebeck effect named after Thomas Seebeck, who discovered it in 1821 and which describes the voltage created whenever two dissimilar metals touch. The contact point produces a small temperature dependent open-circuit voltage [Seebeck voltage]. Although this voltage is nonlinear with respect to temperature at small enough temperature changes, the voltage is approximately linear according to the formula ΔV=SΔT where ΔV is the change in voltage, S is the Seebeck coefficient, and ΔT is the change in temperature. However, due to the fact that S varies with changes in temperature, the output voltages of thermocouples are nonlinear over their operating ranges.
There are several different types of thermocouples, each typically designated by a letter that indicates the composition of the thermocouple according to American National Standards Institute (ANSI) conventions. For example, a J-type thermocouple has one iron conductor and one constantan (a copper-nickel alloy) conductor.
Thermocouple vacuum gauges are used to measure pressure in vacuum systems in the range of approximately 0.001 to 1 Torr. Below this pressure range the thermal conductivity of air is very small while above this range the thermal conductivity is essentially constant. Within this pressure range the thermal conductivity of air increases with increasing pressure and thus can be used to measure the pressure. Gases other than air have a similar characteristic but the pressure-conductivity relation may be somewhat higher or lower. A thermocouple vacuum gauge, [TC gauge] in its most basic configuration, operates by passing a current through a wire, e.g. a nickel wire, so that it gets hot. By connecting another wire of dissimilar metal, e.g. copper, to the center of the nickel wire where it is hottest, an EMF is generated at the ambient ends of the Ni and Cu wires due to the thermocouple effect (Seebeck effect). Assuming that a constant voltage is applied to the ends of the Ni wire then the EMF that is measured can be correlated to temperature, or, since the temperature is dependent on the pressure of the gas around it between about 0.001 and 1 Torr, the EMF can be correlated to pressure. Historically it has been most common to use AC voltage to heat the hot wire.
Zettler, U.S. Pat. No. 4,579,002 discloses a thermocouple vacuum gauge for measuring pressure in an evacuated enclosure. FIG. 1 of that patent shows a block diagram of the gauge. A time-multiplexed servomechanism 10 is used to supply a duration modulated constant amplitude heating pulse to thermocouple 12. In the intervals between heating, the EMF of the thermocouple is measured and compared to a reference voltage. The current needed to maintain the thermocouple at a constant temperature determines the duty cycle of the pulses. This duty cycle is a function of the pressure in the apparatus. Only one thermocouple is employed in this system. A linear response of temperature vs. pressure over approximately six orders of magnitude of pressure is possible with this configuration. A similar type of device is described in Scholl, U.S. Pat. No. 4,633,717.
The background section of Drubetsky, U.S. Pat. No. 5,351,551 has good descriptions of related prior art. The object of this patent is to extend the pressure range of the gauge above 1 Torr by using gas convection from a heated lower wire to an upper thermocouple.
Prior art TC use alternating current to obtain a signal voltage from the TC. It is an object of this invention to provide a simpler direct current voltage to heat the sensor wire.